Conventionally, image formation by use of a thermal transfer recording medium is known, wherein the thermal transfer recording medium is heated with a thermal head, causing imagewise transfer of the ink to the receiving medium to form the image. This method is typically used the preparation of labels, such as name plates.
A thermal transfer recording medium is required to have satisfactory heat sensitivity. Furthermore, when the recording medium is used in an environment containing an organic solvent, such as xylene, acetone or toluene, it is necessary that the image transferred to the label be stable and not eliminated by the presence of the organic solvent.
It has been proposed to add a resin of a specific type to the ink layer and receiving layer in order to obtain superior solvent resistance of the transferred image. For example, an ink layer and receiving layer containing a polyolefin is shown in Japanese Laid Open Patent Publication No H07-5810. The addition of nylon to the ink layer and the receiving layer is shown in Japanese Laid-Open Patent Publication No. H04-347688 and Japanese Laid-Open Patent Publication No. 2001-199171. In Japanese Laid-Open Patent Publication No. H08-230341, an impression thermal transfer recording medium is disclosed which uses an ink layer and receiving layer containing a polyester as the main component.
However, the solvent resistance of recorded images in these is not sufficient.
Another proposed solution provides that the ink layer comprises a metal salt of ethylene-methacrylic acid copolymer. Methacrylic acid has a structure that forms a bridge between the molecule chains by the cation of the metal. When it is heated, the ion bridge becomes poor, and becomes flexible and an ionic bond becomes strong at the time of non-heating, and becomes stronger. Therefore the softening point of the metal salt of ethylene-methacrylic acid copolymer is low, in the temperature range of from 55° C. to 70° C. However, it has excellent solvent resistance.
The use of a metal salt of ethylene-methacrylic acid copolymer in a thermal transfer recording medium has been previously proposed. (Japanese Laid-open Patent Publication No.S63-130385, Japanese Laid-open Patent Publication No.S63-309493, Japanese Laid-open Patent Publication No. H05-77562 and Japanese Laid-open Patent Publication No. H08-230341.) However, the solvent resistance of the recorded image is not sufficient in such cases, particularly with respect to solvents such as xylene, acetone and toluene.